A number of different modulation techniques are commonly used in modern digital communication systems, one of which is known as phase/shift keying (PSK) in which digital information is transmitted as the sequential transmission of carrier pulses of constant amplitude, angular frequency and duration, but of different relative phase. Two common types are differential quadrature phase shift keying (DQPSK) in which information is represented by the phase transitions between carrier pulses, and coherent quadrature phase shift keying (CQPSK) in which a phase reference is provided in the receiver so that the receiver may be phase-synchronized with the transmitter and information may be represented by the amplitude phase of each pulse. Each of these modulation systems is well known in the art and need not be described in further detail herein.
In a QPSK demodulator, there must be two RF inputs and two IF outputs. FIG. 1 is a brief diagram of a basic QPSK phase detector. The phase detector includes two RF input ports 10 and 12, an in-phase power splitter 14, a 90.degree. 3 dB hybrid coupler 16 and two mixers 18 and 20. The IF.sub.1 and IF.sub.2 outputs of the mixers 18 and 20, respectively, will be at the same frequency, which frequency will be equal to the difference between the frequencies of RF.sub.2 and RF.sub.1. For CQPSK or DQPSK, RF.sub.1 and RF.sub.2 will have the same frequency with RF.sub.2 being used as a phase reference signal for the modulated carrier RF.sub.1. RF.sub.1 will be split by the in-phase power splitter 14 and supplied as one input to each of the mixers 18 and 20. The RF.sub.2 signal, however, will be supplied through the coupler 16 and, as is well known in the art, the inputs supplied from the coupler 16 to the mixers 18 and 20 will be 90.degree. out-of-phase. The result will be two orthogonal bit streams IF.sub.1 and IF.sub.2 from which the original information can be recovered.
In mixer applications, IF.sub.1 and IF.sub.2 will typically be combined in a 90.degree. 3 dB hydrid coupler so that the upper (signal) and lower (image) sidebands can be physically separated without the use of filters. Such a configuration is shown in FIG. 2 in which the IF outputs of mixers 18 and 20 are combined in a coupler 22. In both FIGS. 1 and 2, it can be seen that the RF inputs are separated by IF outputs, and vice versa. Thus, in order to combine the IF outputs in a coupler such as shown in FIG. 2, at least one of the IF signal paths must cross the path of the signal RF.sub.2 supplied to the phase detector input 12. In some circuit applications, this requirement may result in a significant disadvantage.
In many applications, the QPSK demodulator will be formed as a microwave integrated circuit (MIC). In fabricating such circuits, it would be desirable from the point of view of manufacturing ease and circuit reliability to form the entire MIC in a single plane. However, if the IF outputs are to be combined in a coupler, the RF-IF-RF-IF arrangement of the input and output ports will require that at least one signal, usually one of the IF signals, leave the plane of the MIC assembly. This will require the use of holes, connectors, cables, vias, or some combination thereof. Not only does this result in difficulties in circuit fabrication, but it may detract from the circuit performance at very high frequencies. G. P. Kurpis and J. J. Taub, "Wideband X-band Microstrip Image Rejection Balance Mixer", IEEE, MTT Symposium Digest 1970, pp. 200-205, describes a typical image rejection receiver in which both IF.sub.1 and IF.sub.2 are routed to the underside of the MIC assembly for processing by the 90.degree. hybrid at IF. J. B. Cochrane and F. A. Marki, "Thin-film Mixers Team Up To Block Out Image Noise", Microwaves, March, 1977, pp. 34-40, later describe a receiver in which double-sided MIC's are used to obtain physical separation of the upper and lower side bands.